Protein switches hold great promise as selective protein therapeutics and as biosensors for diagnostic and basic science applications; however, the creation of such proteins has proven difficult. We propose to create protein switches for these applications using a combinatorial method we developed that views all existing proteins as potential input and output modules for the desired switch. We will create a protein switch that will activate a prodrug only in cancer cells. Such a switch would have potential as a targeted therapeutic for the treatment of cancer. In addition, we will create switches to be used as sensors for kinases in vitro and in live cells. Switches to be used as sensors will be developed on a platform such that sensors for other proteins will be more readily created. Experiments proposed will also inform the study of how best to create switches